1. Field of the Invention
The present invention relates to a semiconductor package, such as a surface-mounted semiconductor package, which is mounted on the surface of a printed circuit board with input/output pins not inserted thereinto, and its manufacturing method.
2. Description of the Prior Art
Recently, a more sophisticated function of a semiconductor package, such as an IC package built into electronic devices including personal computers or the like, requires more numbers of input/output pins. As a matter of fact, the number of input/output pins per package has reached a few hundred.
One of those packages is a Quad Flat Package (QFP), where the input/output pins are arranged on four sides of the square package. However, as more and more input/output pins are used, the package must be increased in size to allow more pins to be arranged on its sides, thus requiring more space.
In order to arrange more pins in a limited space on this type of package, reduction of the pin-to-pin pitch is necessary. However, when the package is mounted on the printed circuit board, for example, by soldering, the input/output pins to be densely arranged sometimes causes electric shorts therebetween.
To solve this problem, a Ball Grid Array (BGA), shown in FIGS. 10, 11(a), and 11(b), has been introduced recently as a new type of package to allow more input/output terminals to be placed.
FIG. 10 is a cross sectional view of a conventional surface-mounted semiconductor package of a BGA type, and FIG. 11(a) and 11(b) are the enlarged sectional views of the part of the semiconductor package surrounded by the rectangle indicated by the symbol A in FIG. 10.
On the semiconductor package shown in FIG. 10, the semiconductor device 1 is attached by bonding, using, for example, adhesives, in a predetermined area on the substrate 2, which includes the stacked fibers 2a such as glass fibers bound by epoxy resin, as shown in FIG. 11(a) and FIG. 11(b). The electrode pads on the semiconductor device 1 are connected via the bonding wires 3 to the conductive patterns 4 on the substrate 2, and the semiconductor device 1 and the bonding wires 3 are sealed with the resin material 5.
On the bottom of the substrate 2 are formed the conductive patterns 6 which correspond to the conductive patterns 4. The corresponding conductive patterns 4 and 6 are electrically connected through the cylindrical conductive coatings 7 formed on the inside walls of the through-holes in the substrate 2. With both of-the semiconductor device 1 and the bonding wires 3 being sealed with the resin material 5, the bumps 8 to be made of material such as solder, are formed on the conductive pattern 6. The bumps 8, which serve like the input/output pins of the QFP, fix the semiconductor device 1 on the printed circuit board 9b, which an electric connection therebetween are performed.
Though the input/output pins of the QFP arranged in line on every side of the package, the bumps 8 of the BGA are placed on the bottom surface of the package in a matrix form. This placement of the BGA solves the problems of the QFP because the pitch between the bumps 8 may be widened.
However, the conventional surface-mounted semiconductor packages of the BGA type, has a drawback in that the water 9 easily pervades the substrate 2 composed of stacked fiber 2a, causing the substrate 2 to be crack.
In particular, once the bumps 8 are heated, so as to melt the bumps, when the package is mounted on the printed circuit board 9b, the water 9 lying in the substrate 2 expands rapidly through the heat of the bumps 8, sometimes causing a crack 9a to form in the substrate 2, as shown in FIG. 11(b). If the crack 9a reaches the cylindrical conductive coating 7 positioned near an edge of the substrate 2, the cylindrical conductive coating 7 is broken, thus breaking the electric connection.